Failsafe control circuit for electrical appliances

1. A failsafe control circuit for electrical appliances whereby at least one electrical load ( 10 ) is activated, the control circuit comprising logic control means ( 1 ), operation switching means ( 8 ) between said electrical load ( 10 ) and a supply voltage (V), and a first drive circuit ( 6 ) which receives a first command signal ( 4 ) from the logic control means ( 1 ), said first drive circuit ( 6 ) acting on said operation switching means ( 8 ), security switching means ( 9 ) between the electrical load ( 10 ) and the supply voltage (V), and a second drive circuit ( 7 ) which receives a second command signal ( 5 ) from the logic control means ( 1 ), said second drive circuit ( 7 ) acting on the security switching means ( 9 ) so that it closes said security switching means ( 9 ) only if said second command signal ( 5 ) is a pulse signal.

2. The failsafe control circuit according to claim 1 , wherein the security switching means ( 9 ) comprise a power relay with a coil (K 2 ), and the drive circuit ( 7 ) comprises a monostable circuit which acts on said coil (K 2 ), so that it only permits a current to flow through the coil (K 2 ) of the security switching means ( 9 ) if the second command signal ( 5 ) is a pulse signal at a frequency higher than a pre-established minimum frequency.

3. The failsafe control circuit according to claim 1 , wherein at least one of the switching means ( 8 , 9 ) comprises a triac.

4. The failsafe control circuit according to claim 1 , wherein it also comprises security switching means ( 13 ) between the first and second drive circuits ( 6 , 7 ) and its supply voltage (V 2 ), and comprises a third drive circuit ( 12 ) which receives a third command signal ( 11 ), so that said drive circuit ( 12 ) closes said security switching means ( 13 ) only if the third command signal ( 11 ) is a pulse signal.

5. The failsafe control circuit according to claim 1 , wherein a maximum operating time is set, after which the electrical load ( 10 ) is no longer energized.

6. The failsafe control circuit according to claim 2 , wherein the monostable circuit comprises a transistor connected in series with the coil (K 2 ) and a capacitor (C 1 ) which filters the second command signal ( 5 ) before it reaches said transistor.

7. The failsafe control circuit according to claim 2 , wherein the operation switching means ( 8 ) comprise a power relay with a coil (K 1 ), and the drive circuit ( 6 ) comprises a transistor (T 1 ) connected in series with said coil (K 1 ).

8. The failsafe control circuit according to claim 5 , wherein the maximum operating time is determined in accordance with the power that is being supplied to the electrical load ( 10 ).

9. The failsafe control circuit according to claim 7 , wherein the first command signal ( 4 ) is also a pulse signal.

10. The failsafe control circuit according to any of claims 1 , 2 or 6 , wherein the operation switching means ( 8 ) comprise a triac.

11. The failsafe control circuit according to claim 6 , wherein the operation switching means ( 8 ) comprise a power relay with a coil (K 1 ), and the drive circuit ( 6 ) comprises a transistor (T 1 ) connected in series with said coil (K 1 ).

12. The failsafe control circuit according to claim 9 , wherein there is a phase difference between the first command signal ( 4 ) and the second command signal ( 5 ), so that the overlap of energy which has to be supplied to coil (K 1 ) and coil (K 2 ) is reduced.

13. The failsafe control circuit according to claim 11 , wherein the first command signal ( 4 ) is also a pulse signal.

14. The failsafe control circuit according to claim 13 , wherein there is a phase difference between the first command signal ( 4 ) and the second command signal ( 5 ), so that the overlap of energy which has to be supplied to coil (K 1 ) and coil (K 2 ) is reduced.